Relevance of Research Plan: One priority area for the Department of Veterans Affairs (DVA) is to provide quick, effective, recovery-oriented treatments for depressive/anxiety disorders, representing the most common and impairing psychiatric disorders in Veterans. Cognitive behavioral therapy (CBT) has demonstrated reliable efficacy in treating depressive/anxiety disorders and their related impairments. However, CBT is underutilized due in large part to the complexity and number of disorder-specific treatments (DSTs) protocols across disorders, as well as extensive training needed to apply them to different diagnoses and different combinations of diagnostic comorbidities. Recently, Transdiagnostic Behavior Therapy (TBT) was developed as a transdiagnostic intervention to address each of the depressive/anxiety disorders and their comorbidities in Veterans within a single treatment protocol. Initial studies of TBT, including a funded CSR&D Career Development Award (CDA), demonstrate that the treatment is efficacious in reducing psychiatric symptomatology and related impairments in Veterans presenting with a wide range of diagnoses. Preliminary findings also support the ease of dissemination and implementation of TBT by DVA providers. Although additional study is needed with larger samples and improved comparison groups, the transdiagnostic approach in TBT may address many of the current limitations of the DSTs discussed above. Primary Aims in Research Plan: The proposed study aims to test the non-inferiority of TBT and DSTs in improving psychiatric symptomatology and related impairments. It is hypothesized that TBT will result non-inferior improvements in psychiatric symptomatology and related impairments compared to the DST comparison groups. A secondary aim is to compare the feasibility and acceptability of TBT and DSTs. Participant population: The participants of the proposed study will include 216 Veterans with posttraumatic stress disorder (PTSD), major depressive disorder (MDD), and/or panic disorder and agoraphobia (PD/AG). General inclusion criteria involve: 1) a diagnosis of a PTSD, MDD, or PD/AG; and 2) a willingness and competence to provide informed consent for research participation. Procedures to be used: A randomized controlled trial (RCT) with TBT and DSTs conditions will be completed. Participation in the RCT will involve completion of: 1) consent documentation, 2) intake assessment, 3) random assignment to treatment condition, 4) weekly 45-60-minute appointments of psychotherapy for 12 weeks, and 5) self-report assessments of psychiatric symptomatology and impairment at intake, mid- treatment, post-treatment, and 6-month follow-up. Process variables also will be investigated. Anticipated Impacts on Veterans Health Care: Upon investigation with supported efficacy and non-inferiority as compared to DSTs, TBT will represent an excellent treatment option within the DVA in terms of providing a single protocol that is easy to disseminate to and implement by providers as well as being efficacious in addressing psychiatric symptomatology and related impairments in Veterans with various depressive/anxiety disorders and related comorbidities.